Optical measuring method and device for measuring a magnetic alternating field with an expanded measuring range and good linearity

ABSTRACT

Linearly polarized measuring light is split after traversing a Faraday sensor device into two partial light signals having planes of polarization inclined at 45°. A measured signal, which is proportional to the tangent of the Faraday rotation angle, is derived from the two partial light signals.

FIELD OF THE INVENTION

The present invention relates to a method and an arrangement formeasuring an alternating magnetic field. An alternating magnetic fieldis understood to be a magnetic field which has in its frequency spectrumonly frequency components differing from zero.

BACKGROUND OF THE INVENTION

Optical measuring arrangements for measuring an electrical current in anelectrical conductor are known which are based on the magneto-opticFaraday effect, and are therefore also designated as magneto-opticcurrent transformers. In a magneto-optic current transformer, linearlypolarized measuring light is transmitted through a Faraday sensor devicewhich is arranged in the vicinity of the electrical conductor andincludes an optically transparent material exhibiting the Faradayeffect. Because of the Faraday effect, the magnetic field generated bythe current causes a rotation of the plane of polarization of themeasuring light by a rotational angle ρ, which is proportional to thepath integral over the magnetic field along the path covered by themeasuring light in the sensor device. The constant of proportionality isthe Verdet constant V. The Verdet constant V is generally a function ofthe material and the temperature of the sensor device, as well as of thewavelength of the measuring light employed. In general, the sensordevice surrounds the electrical conductor, so that the measuring lightruns at least once around the electrical conductor in a virtually closedpath. The rotational angle ρ is, in this case, essentially directlyproportional to the amplitude I of the current to be measured, inaccordance with the relation

    ρ=N·V·I                              (1),

N being the number of revolutions of the measuring light around theelectrical conductor. The Faraday rotational angle ρ is determinedpolarimetrically by performing a polarization analysis of the measuringlight running through the sensor device, in order to obtain a measuringsignal for the electrical current.

It is known for the purpose of polarization analysis to use an analyzerto decompose the measuring light, after it has traversed the sensordevice, into two linearly polarized light components L1 and L2 havingplanes of polarization, which are directed perpendicularly with respectto one another. A polarizing beam splitter can be used as the analyzerfor this polarization analysis. Specifically, some of the types ofpolarizing beam splitters that can be used in this analysis include aWollaston prism or a simple beam splitter having two downstreampolarizers whose axes of polarization are rotated by π/2 or 90° withrespect to one another. Each of the two light components L1 and L2 isconverted by one assigned photoelectric transducer into, in each case,an electrical intensity signal T1 or T2, which is proportional to thelight intensity of the light component L1 or L2, respectively. Ameasuring signal

    T=(T1-T2)/(T1+T2)                                          (3)

which corresponds to the quotient of a difference and the sum of the twointensity signals T1 and T2, as described in PCT Application No. WO95/10046, is formed from these two electrical signals.

Disregarding interference effects, this measuring signal T is given by

    T=sin(2π+ζ)=sin(2·N·V·I+ζ)(4),

ζ being an offset angle for I=0 A, which is a function of the anglebetween the plane of polarization of the measuring light on beingcoupled into the Faraday element and a distinctive intrinsic opticalaxis of the analyzer.

Although, according to equation (1), the Faraday measuring angle ρ isitself a linear, and thus unique, function of the current I, accordingto equation (4) the measuring signal T is a unique function of themeasuring angle ρ only over an angular range of at most π/2 (or 90°).Consequently, it is possible using these polarimetric magneto-opticcurrent transformers to measure uniquely only those electrical currentswhich lie in a current measuring range (current measuring interval) MRwith an interval length of

    |MR|=π/(2·N·V)      (5)

It is clear from equation (5) that the magnitude |MR| of the currentmeasuring range MR of a magneto-optic current transformer can be set bythe selection of materials having different Verdet constants V for theFaraday element and/or by the number N of revolutions of the measuringlight around the electrical conductor. A larger current measuring rangeis obtained by setting the product N·V in the denominator smaller.However, such a selection of a larger current measuring range MR isinescapably attended by a reduced measuring resolution MA of the currenttransformer for a given display resolution. The measuring resolution MAis defined in this case as the absolute value |MS| of the measuringsensitivity MS of the current transformer. The measuring sensitivity MScorresponds to the gradient of the characteristic curve of themagneto-optic current transformer at an operating point, and in the caseof two-channel evaluation, is given according to equation (4) by

    MS=dT/dI=2·N·V·cos(2·N·V.multidot.I+ζ)                                              (6).

It is immediately evident from equation (6) that reducing the productN·V leads, in the case of both evaluation methods, to a reduction in themeasuring resolution MA=|MS|.

European Patent Application No. 088 419 describes a magneto-opticcurrent transformer in which two Faraday glass rings, which are made ofFaraday materials having different Verdet constants and thus each havinginherently different current measuring ranges, are arranged parallel toone another about a common electrical conductor. Each Faraday glass ringis assigned a transmission unit for transmitting linearly polarizedmeasuring light into the glass ring and a two-channel evaluation unitfor calculating a respective measuring signal for each Faradayrotational angle. The two measuring signals of the two evaluation unitsare fed to an OR gate, which determines a maximum signal from the twomeasuring signals. This maximum signal is used to switch between themeasuring ranges of the two glass rings. Different measuring ranges ofthe two glass rings can also be obtained given the same glass materialfor the two glass rings by employing measuring light of differentwavelengths. In this context, the wavelength dependence of the Faradayrotation is utilized.

The publication entitled "Fiber Optic Current Sensor With Optical AnalogTransmission", SENSOR 93 Conference Report IV Vol. 11.1, pages 137 to144, describes a magneto-optical current transformer for protectivepurposes for measuring alternating currents, in which, after traversinga Faraday optical fiber, linearly polarized light is split into twopartial light signals and each of these light signals is fed to ananalyzer. The intrinsic axes (axes of polarisation) of the two analyzersare directed at an angle of 45° or 58° is relative to one another. Thelight intensities passed by the analyzers are not normalized untildivision by their constant components, which are obtained by peak valuerectification. Subsequently, a product of the normalized signals isformed and this product is then differentiated. The Faraday rotationalangle is obtained directly by integration. As a result, a signal isobtained which is proportional to the current and, therefore, is notsubject to measuring range limitations. However, this method iscomparatively costly.

European Patent No. 208 593 describes a magneto-optic currenttransformer in which, after traversing a Faraday optical fibersurrounding an electrical conductor, linearly polarized measuring lightis split by a beam splitter into two partial light signals and each ofthese partial light signals is fed to an analyzer. The intrinsic axes ofthe two analyzers are directed at an angle of 0° and 45°, respectively,relative to the coupling polarization of the measuring light. Thisproduces a first, sinusoidal signal at the output of one analyzer, and asecond, cosinusoidal signal at the output of the other analyzer. Thesetwo signals are, in each case, non-unique, oscillating functions of thecurrent in the electrical conductor, which are phase-shifted withrespect to one another by an angle of 90°. A unique measuring signal isnow composed from these two non-unique signals by comparing the sign andthe absolute values of the measuring values of the first, sinusoidalsignal and of the second, cosinusoidal signal. As soon as the absolutevalues of the sine and cosine are equal, that is to say given anintegral multiple of 45°, a switch is made, as a function of the sign ofsine and cosine, from a unique branch of the first, sinusoidal signal toa unique branch of the second, cosinusoidal signal, or vice versa. Themeasuring range of this known magneto-optic current transformer is thus,in principle, unlimited. However, the method is an incremental method,with the result that the operating point for current zero must be resetanew whenever there is a failure of the electronics of the currenttransformer.

SUMMARY OF THE INVENTION

An object of the present invention is to specify a method and anarrangement, having an extended measuring range and good linearity, formeasuring an alternating magnetic field.

In order to achieve this object, linearly polarized measuring light iscoupled into a sensor device which exhibits the Faraday effect and isarranged in the alternating magnetic field, at least during themeasuring operation. The measuring light traverses the sensor device atleast once and is thereafter fed into two linearly polarized partiallight signals whose directions of polarization are directed, relative toone another, at an angle of essentially an odd multiple of 45° or π/4.The two partial light signals are, in each case, converted into anelectrical intensity signal which is a measure of the light intensity ofthe associated partial light signal. An alternating signal component anda constant signal component are determined from a first of the twoelectrical intensity signals, and a constant signal component isdetermined from a second of the two intensity signals. The alternatingsignal component contains essentially all the frequency components ofthe alternating magnetic field. The two constant signal components, incontrast, contain essentially no frequency components of the alternatingmagnetic field. A measuring signal is now derived for the alternatingmagnetic field and is proportional to a quotient of twointensity-normalized signals, a first of the two intensity-normalizedsignals corresponding to the quotient of the alternating signalcomponent and the constant signal component of the first intensitysignal, and a second of the two intensity-normalized signalscorresponding to the quotient of the second intensity signal and theconstant signal component thereof. This measuring signal is, on the onehand, virtually independent of undesirable intensity fluctuations of themeasuring light and is, on the other hand, a unique function over anangular range of approximately π for the Faraday rotational angle ρ, bywhich the plane of polarization of the measuring light in the sensordevice is rotated based on the magnetic field, for example over the openangular range of T/2<ρ<+π2 . Furthermore, the measuring signal has anexcellent linearity in a large range, around an operating point situatedin the middle of the measuring range.

According to the present invention, as a measure of the root-mean-squarevalue of the alternating magnetic field, a root-mean-square value isformed from the measuring signal, for the purpose of precisionmeasurement.

In order to measure an alternating electrical field, the sensor deviceis arranged in the alternating magnetic field generated inductively bythe alternating current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an arrangement for measuring an alternatingmagnetic field and, in particular, for measuring an alternatingelectrical current in accordance with an exemplary embodiment of theinvention.

FIG. 2 illustrates the root-mean-square value of the measured current asa function of the Faraday measuring angle.

DETAILED DESCRIPTION

Represented in FIG. 1 is an exemplary embodiment of an arrangement formeasuring an alternating magnetic field H which can be generated, inparticular, by an electrical current I in an electrical conductor 2. Asensor device 3 exhibiting the magneto-optic Faraday effect is arrangedin alternating magnetic field H. In the embodiment represented, sensordevice 3 is formed by means of a single-mode optical fiber whichpreferably surrounds electrical conductor 2 in the form of a measuringwinding of at least one turn. It is preferred to provide an annealedoptical fiber which is distinguished by low linear birefringence andvirtually negligible circular birefringence. However, sensor device 3can also be formed from one or more solid bodies, preferably made fromglass, exhibiting the Faraday effect, and can, in particular, surroundelectrical conductor 2 as a polygonal annular body. Means are providedfor coupling linearly polarized measuring light L into sensor device 3.The direction of polarization of the electrical field strength vector ofmeasuring light L during coupling into sensor device 3 is denoted belowas the direction of coupling polarization of measuring light L. Themeans for coupling measuring light L into sensor device 3 can, asrepresented, contain a light source 9 and a polarizer 10 for linearlypolarizing the light of light source 9, or also a light source which isitself linearly polarized, such as a laser diode, for example. In theembodiment represented, the axis of polarization (transmission axis) ofpolarizer 10 prescribes the direction of coupling polarization ofmeasuring light L. The linearly polarized measuring light L coupled intosensor device 3 traverses sensor device 3 and is fed, after traversingsensor device 3, to a beam splitter 4. Beam splitter 4 decomposesmeasuring light L into two light components L1' and L2' having the samepolarization. For example, beam splitter 4 can be formed with asemi-transparent mirror inclined at an angle of preferably 45° to thedirection of propagation of measuring light L. A first polarizer 5,which forms a first partial light signal L1 projected onto its axis P1of polarization, is arranged in the optical path (beam path) of firstlight component L1'. A second polarizer 6, which forms a second partiallight signal L2 projected onto its associated axis P2 of polarization,is arranged in the optical path of second light component L2'. Axis P1of polarization of first polarizer 5 and axis P2 of polarization ofsecond polarizer 6 form an angle of at least approximately

    α=(2n+1)·45° or

    α=(2n+1)·(π/4)                           (7)

relative to one another, where n is a whole number. Axis P1 ofpolarization of first polarizer 5 is preferably directed at an angle ofat least approximately +45° or +π/4, or -45° or -π/4 relative to thedirection of coupling polarization of measuring light L, and axis P2 ofpolarization of second polarizer 6 is directed at an angle of 0° or 0relative to the direction of coupling polarization of measuring light L.

The two component signals L1 and L2 are fed to an assigned photoelectrictransducer 7 or 8, respectively. Each photoelectric transducer 7 and 8converts associated light signals L1 and L2, respectively, into anelectrical intensity signal S1 or S2, respectively, which is a measureof the intensity of the respective partial light signal L1 or L2.Generally, electrical intensity signal S1 or S2 is proportional to thelight intensity of associated partial light signal L1 or L2,respectively. The output of first photoelectric transducer 7 is thenelectrically connected to the input on a high-pass filter 11 and to theinput of a low-pass filter 12. High-pass filter 11 forms an alternatingsignal component A1 of first intensity signal S1,and low-pass filter 12forms a constant signal component D1 of this first intensity signal S1.The separating frequency of high-pass filter 11 and low-pass filter 12are set such that alternating signal component A1 contains all thefrequency components of alternating magnetic field H, and constantsignal component D1 is independent of alternating magnetic field H.Alternating signal component A1 of first intensity signal S1 is fed froman output of high-pass filter 11 to a first input of a divider 14.Constant signal component D1 of first intensity signal S1 is fed from anoutput of low-pass filter 12 to a second input of divider 14.

Divider 14 now forms quotient signal A1/D1 of alternating signalcomponent A1 to constant signal component D1 of first intensity signalS1. This quotient signal A1/D1 is an intensity-normalized signal, thatis to say it is independent of changes in the intensity of measuringlight L, for example, owing to intensity fluctuations of light source 9or attenuation losses in the light path of measuring light L or of firstpartial light signal L1. The output of second photoelectric transducer 8is electrically connected to the input of a low-pass filter 13 and to afirst input of a divider 15. Lowpass filter 13 forms a constant signalcomponent D2 of second intensity signal S2. The separating frequency oflowpass filter 13 is set such that constant signal component D2 containsno frequency components of alternating magnetic field H. There is nowpresent at an output of divider 15 a quotient signal S2/D2 whichcorresponds to the quotient of second intensity signal S2 and constantsignal component D2 thereof. This quotient signal S2/D2 is also anintensity-normalized signal, and is thus independent of changes inintensity in measuring light L and in second partial light signal L2.Since changes in intensity in the light paths of two partial lightsignals L1 and L2 are compensated by the intensity normalization,multimode fibers can also be used to transmit two partial light signalsL1 and L2. Two normalized signals A1/D1 and S2/D2 are now fed, in eachcase, to an input of a further divider 16. Divider 16 forms the quotientof two normalized signals A1/D1 and S2/D2 as measuring signal

    M=(A1/D1)/(S2/D2)                                          (8)

which can be tapped at an output 30 of the arrangement.

This measuring signal M is similar to function tan(ρ) of Faradayrotation angle ρ by which the direction of polarization (plane ofpolarization) of measuring light (L) is rotated in sensor device 3 basedon alternating magnetic field H. However, tangent function tan(ρ) is aunique function of rotational angle ρ over an angular interval having aninterval length of approximately π, specifically for

-π/2+2mπ<ρ<+π/2 +2mπ, with m being a whole number. The result is ameasuring range which is virtually twice as large as in the case of themeasuring signals obtained in accordance with the related art, which areproportional to sin(2ρ).

In a preferred embodiment, means 17 are provided for formingroot-mean-square value M_(eff) of measuring signal M, which functions asa measure of the amplitude (absolute value) of alternating magneticfield H, or as a measure of root-mean-square value I_(eff) of anelectrical current I in electrical conductor 2. FIG. 2 showsroot-mean-square value M_(eff) of measuring signal M for a sinusoidalelectrical current I=2⁰.5 I_(eff) sin (ωt), plotted over an angularrange of 0° to approximately 60° of root-mean-square value ρ_(eff) ofFaraday rotational angle ρ-2⁰.5 ρ_(eff) sin (ωt). Root-mean-square valueI_(eff) of electrical current I is then obtained from equation ρ_(eff)=2 NV I_(eff) with N the number of turns of the fiber coil (measuringwinding) and Verdet constant V of sensor device 3. Any analog or digitalcircuit known per se can be used to form root-mean-square value M_(eff).

Root-mean-square value M_(eff) of measuring signal M can also besubjected to a subsequent linearization, preferably with the aid of adigital signal processor 18. Linearized root-mean-square value M_(eff)^(lin), then linearly dependent on rotational angle ρ, is applied to anoutput 20. Of course, root-means-quare value M_(eff) itself can also beapplied to an output (not represented).

Instead of high-pass filter 11, it is also possible, for the purpose offorming alternating signal component A1 of first intensity signal S1, toprovide a subtractor which forms difference S1-D1 between firstintensity signal S1 and its constant signal component D1, which isformed by lowpass filter 12. This difference corresponds precisely toconstant signal component A1. Conversely, instead of low-pass filter 12,it is also possible, for the purpose of forming constant signalcomponent D1 of first intensity signal Si, to provide a subtractor whichforms difference S1-A1 between first intensity signal S1 and itsalternating signal component A1, which is formed by high-pass filter 11.This difference corresponds precisely to constant signal component D1.Furthermore, low-pass filter 13 can also be replaced by a high-passfilter for the purpose of forming an alternating signal component A2 ofsecond intensity signal S2, and by a subtractor for the purpose offorming constant signal component D2 of second intensity signal S2 bysubtracting alternating signal component A2 from second intensity signalS2. Finally, the analog filters represented can also be replaced bydigital filters and analog-to-digital convertors connected upstream.

Of course, instead of analog dividers 14, 15 and 16, it is also possibleto provide digital calculating means as arithmetic means for derivingmeasuring signal M, in accordance with relation (8). As explained above,the value of relation (8) is based on alternating signal component A1and constant signal component D1 of first intensity signal S1 and fromsecond intensity signal S2 and a constant signal component D2 thereof.Such a digital calculating means may include a microprocessor or adigital signal processor having an analog-to-digital converter connectedupstream. It is preferable to provide both digital filters and digitalarithmetic means. The analog-to-digital conversion is then performedupstream of the digital filters.

The optical coupling of the various optical components of the measuringarrangement is preferably supported by collimator lenses (Grin lenses),not depicted, for focusing the light.

Instead of the type of transmission shown in FIG. 1, in which measuringlight L traverses sensor device 3 only once, it is also possible toprovide an arrangement of the reflection type in which, after traversingsensor device 3 a first time, measuring light L is retroflected intosensor device 3 with the aid of a mirror and traverses sensor device 3 asecond time in the opposite direction, before it is fed to beam splitter4.

What is claimed is:
 1. A method for measuring an alternating magneticfield, comprising the steps of:a) coupling a linearly polarizedmeasuring light into a sensor device, the sensor device being arrangedin the alternating magnetic field and exhibiting the Faraday effect; b)causing the linearly polarized measuring light to traverse the sensordevice at least once; c) splitting the linearly polarized measuringlight into a first linearly polarized partial light signal and a secondlinearly polarized partial light signal, wherein a direction ofpolarization of the first linearly polarized partial light signal and adirection of polarization of the second linearly polarized partial lightsignal are at an angle relative to one another, the angle beingsubstantially an odd multiple of 45°; d) converting the first linearlypolarized partial light signal into a first electrical intensity signal,the first electrical intensity signal being a measure of a lightintensity of the first linearly polarized partial light signal; e)converting the second linearly polarized partial light signal into asecond electrical intensity signal, the second electrical intensitysignal being a measure of a light intensity of the second linearlypolarized partial light signal; f) determining a first alternatingsignal component and a first constant signal component from the firstelectrical intensity signal, the first alternating signal componentincluding substantially all frequency components of the alternatingmagnetic field; g) determining a second constant signal component fromthe second electrical intensity signal, wherein the first constantsignal component and the second constant signal component includessubstantially none of the frequency components of the alternatingmagnetic field; h) deriving a first intensity-normalized signal from aquotient of the first alternating signal component and the firstconstant signal component; i) deriving a second intensity-normalizedsignal from a quotient of the second electrical intensity signal and thesecond constant signal component; and j) deriving a measuring signal forthe alternating magnetic field, the measuring signal being proportionalto a quotient of the first intensity-normalized signal and the secondintensity-normalized signal.
 2. The method according to claim 1, furthercomprising the step of:forming from the measuring signal a measure of aroot-mean-square value of the alternating magnetic field.
 3. The methodaccording to claim 1, wherein the alternating magnetic field isgenerated by an alternating electrical field, and wherein the methodfurther comprises the step of measuring the alternating electricalfield.
 4. A system for measuring an alternating magnetic field,comprising:a) a sensor device capable of exhibiting the Faraday effect;b) an arrangement for coupling a linearly polarized measuring light intothe sensor device; c) an arrangement for splitting the linearlypolarized measuring light, after the linearly polarized measuring lighthas traversed the sensor device at least once, into a first linearlypolarized partial light signal and a second linearly polarized partiallight signal, a direction of polarization of the first linearlypolarized partial light signal and a direction of polarization of thesecond linearly polarized partial light signal are at an angle relativeto one another, the angle being substantially an odd multiple of 45°; d)an arrangement for converting the first linearly polarized partial lightsignal into a first electrical intensity signal, the first electricalintensity signal being a measure of a light intensity of the firstlinearly polarized partial light signal, the arrangement for convertingfurther for converting the second linearly polarized partial lightsignal into a second electrical intensity signal, the second electricalintensity signal being a measure of a light intensity of the secondlinearly polarized partial light signal; e) an arrangement fordetermining a first alternating signal component and a first constantsignal component from the first electrical intensity signal, the firstalternating signal component essentially including all frequencycomponents of the alternating magnetic field, the arrangement fordetermining further for determining a second constant signal componentfrom the second electrical intensity signal, wherein none of the firstconstant signal component and the second constant signal componentincludes the frequency components of the alternating magnetic field; andf) an arrangement for deriving a first intensity-normalized signal froma quotient of the first alternating signal component and the firstconstant signal component, for deriving a second intensity-normalizedsignal from a quotient of the second electrical intensity signal and thesecond constant signal component, and for deriving a measuring signalfor the alternating magnetic field, the measuring signal beingproportional to a quotient of the first intensity-normalized signal andthe second intensity-normalized signal.
 5. The system according to claim4, further comprising an arrangement for forming from the measuringsignal a measure of a root-mean-square value of the alternating magneticfield.
 6. The system according to claim 4, wherein the alternatingmagnetic field is generated by an alternating electrical field, andwherein the system further comprises an arrangement for measuring thealternating electrical field.